Introduction

A partially evacuated, sealed glass tube containing copper (I) tetraiodomercurate (II) is immersed in a beaker of boiling water.  The compound changes from a bright red to dark brown at 80oC.  When returned to room temperature, the red color reappears.

Equations

At room temperature, the Cu+ and Hg2+ ions are in separate layers, alternating with I- ions and each other.  At high temperature, the metal ions move at random into vacant sites.

Supplies

Assemble the following equipment:

    •   sealed, partially evacuated glass tube of Cu2HgI4
    • hot plate
    • tall 1000mL beaker with ~650mL of water
    • thermometer
    • heat-protective glove

To Conduct Demonstration

  1.  Heat water in a tall 1000mL beaker on a hot plate.
  2. Place the sealed tube of Cu2HgI4 in the beaker.
  3. If desired, measure the temperature of the water bath to determine the approximate temperature at which the color change takes place. 
  4. Remove the tube and allow it to cool.  The red color will return.  Repeat as many times as desired.
  5. Alternate Procedure instead of steps one and two:
  6. Place tube of Cu2HgI4 in a liter tall form beaker of room temperature water and heat, noting the temperature that the compound begins and finishes turning color (70-80 degC).

Demo Time: 10 – 15  minutes if placing tube in boiling water.

                              40 minutes If heating the water with the tube in it.

Safety and Disposal

Wear heat-protective gloves when handling the beaker and tube.

Chemical in the sealed tube is a mercury compound.  Be prepared to contain it and do mercury clean ip if the tube were to break!